1. Field of the Invention
The present invention relates to a driver circuit that controls lighting of an LED backlight of a liquid crystal display device.
2. Description of the Related Art
In a case where a cold-cathode tube (CCFL) is used as a backlight of a liquid crystal display device, a high voltage is necessary for driving the CCFL. For this reason, as illustrated in FIG. 4, an inverter circuit is used for driving the backlight.
As backlights of liquid crystal display devices, more and more LEDs are used. In an LED backlight system, a driver circuit is necessary as a current source used for driving the LED. As a luminance adjustment system in such a case, a PWM system is mainly used. As illustrated in FIG. 5, a PWM signal is input from a control circuit to the driver circuit. The driver circuit controls a drive current for an LED in accordance with the PWM signal input from the control circuit. In one type of LED drive, an LED drive current value is changed and, in another type of LED drive, as illustrated in FIGS. 6A to 6D, PWM control for changing the duty ratio of the driving time is performed as in the case of a PWM signal input with a constant current value.
However, in the case of employing a method in which the backlight is used to have constant luminance without adjusting the luminance, in the conventional CCFL system, the luminance can be adjusted in accordance with the adjustment of an analog signal. However, in the case of the LED type, even in a case where the luminance configured to be constant, it is necessary to use a PWM circuit, which causes an increase in the cost.